The present invention relates to a rescheduling or reset mechanism for use on an aircraft propeller assembly having four or more blades and usually driven by a gas turbine engine. Such aircraft propeller assemblies and control systems are disclosed, for example, in U.S. Pats. No. 2,694,459, 2,986,222, 3,057,140, 3,249,159, 3,380,535, 3,575,529, 4,097,189 and U.S. Pat. No. 4,650,402 which are owned by the Assignee of the present invention and the disclosures of which are herein incorporated by reference.
In general, the pitch of the propeller blades is changed by varying the oil pressure to the hydraulic cylinder and piston projecting forwardly from the propeller hub and connected by crank arms or links to the bases of the propeller blades. Oil from the engine is increased in pressure by a pump in the propeller governor and is supplied to the propeller hub and piston through an axial oil passage in the engine shaft. Usually, the piston is spring biased to a rear position, and the springs cooperate with twisting movements produced by blade counterweights for moving the propeller blades to a feather position. Oil pressure moves the piston forwardly to change the blade pitch through a high pitch and low pitch to a reverse pitch.
In normal in-flight operation, the blade angle or pitch is controlled by the governor at all times. However, when the governor attempts to reduce the blade pitch below a predetermined low or fine pitch setting, the piston picks up a set of axially extending rods of a beta feedback mechanism. The rearward ends of the rods are connected to a feedback collar which receives a sliding carbon block mounted on one end portion of a feedback lever. The lever is pivotally supported by the actuating element of a beta valve connected to control hydraulic fluid to the cylinder and piston. The opposite end of the lever is connected by a power control cable to a pilot operated power lever in the cockpit of the aircraft. The beta valve is designed so that further axial forward movement of the piston and feedback rods and collar will shut off the supply of high pressure oil to the propeller piston, thus preventing any further pitch change beyond the in-flight idle position. However, if the governor senses an overspeed condition, the governor remains effective to reduce the oil pressure which causes the spring to produce an increase in the blade angle or pitch.
In aircraft propellers having four or more blades, there are certain operating placards which restrict operation of the propeller within a predetermined operating range, for example, in a range of 900-1200 rpm, so that the propeller avoids operating in an undesirable reactionless vibration mode. To avoid this mode when the aircraft lands, it is desirable to reset or reduce the pitch of the propeller blades from the inflight idle setting or position to an on-ground idle setting or position so that the propeller unloads the engine and operates at a higher rpm.
Various electrical and/or pneumatic devices have been connected to the beta valve control linkage to produce a resetting or rescheduling of the propeller blades from the in-flight idle position to the ground idle position. The change in the beta valve causes the under-speeding governor to provide sufficiently high pressure oil to the propeller piston for decreasing the blade pitch to the on-ground idle position. However, these devices have limitations on the amount of pitch decrease and are usually difficult to install, adjust and maintain. Another form of blade pitch rescheduling or reset mechanism is disclosed in U.S. Pat. No. 4,643,798 which also discusses the problems associated with operating a four blade propeller within the reactionless mode. The mechanism disclosed in this patent uses a balancing spring and lost motion system which is incorporated on the beta feedback control mechanism.